Overcoming Proterozoic quartzite sand-body miscorrelations: integrated sequence stratigraphy and SHRIMP U–Pb dating of the Surprise Creek Formation, Torpedo Creek and Warrina Park Quartzites, Mt Isa Inlier

Abstract

In the western part of the Mt Isa Inlier one of the most obvious lithostratigraphic correlations is the Torpedo Creek Quartzite (Lawn Hill Platform) and the Warrina Park Quartzite (Leichhardt River Fault Trough). These two sand bodies form the basal lithostratigraphic unit of their respective groups – McNamara and Mt Isa – and crop out as easily mappable, prominent ridges of white quartzite. Correlation of these sand bodies forms the linchpins for lithostratigraphic subdivision in the region. To test this correlation a 2000 m-thick stratigraphic interval around these two quartzite units was analysed using two modern techniques. A sequence stratigraphic analysis, which included detailed surface gamma-ray logging, was undertaken at 20 measured sections over an area of about 18 000 km2 to identify and correlate important stratal surfaces. Complementary high precision U–Pb SHRIMP dating of tuffaceous intervals was also done to provide independent control for the resulting new subdivisions of the section. The sequence analysis and SHRIMP dating of these units and the underlying Surprise Creek Formation demonstrate major miscorrelations in the current lithostratigraphy. SHRIMP dating helps to define a major unconformity with up to 20 million years of missing rock record: rocks above the surface are around 1670–1660 Ma, whilst those below are around 1690 Ma. Although 1:100 000 lithostratigraphic mapping enabled the recognition of an unconformity at the base of the Mt Isa and McNamara Groups, in the majority of cases this surface does not correlate with the major lithostratigraphic boundary with which it was associated. In consequence this unconformity and supersequence boundary remains unrecognised in most areas. The consequences of such miscorrelations are inadequate and inaccurate reconstructions of basin geometry and stratigraphic architecture, and have important economic implications when searching for stratabound base metals.